This invention relates to use of a solar irradiance diffuser panel for measurements from a spacecraft of solar radiation reflected from earth and, more particularly, to a method and apparatus for in-flight calibration of the diffuser panel.
There is interest in the measurement of solar light or radiation reflected from the earth. Apparatus for performing the measurement is carried by a spacecraft. It is convenient to state the measurement of the reflected light in terms of a percentage of the intensity of sunlight incident upon the earth. One way of establishing the percentage reflection is to employ a solar irradiance diffuser panel to reflect incident solar radiation to the measurement apparatus. Thereby, the diffuser panel serves as a diffuse reference source of solar radiation in the measurement process. The measurement apparatus measures the intensity of the reference source, namely, the light reflected from the diffuser panel, and also measures the intensity of light reflected directly from the earth. By comparing the two measurements, the measurement of the intensity of the reflected light from the earth is expressed readily as a fraction or percentage of the intensity of the incident solar radiation as represented by the reflectance of the reference diffuser. Data of the reflected light is transmitted back to a receiving station on the earth.
A problem arises in that the spectral reflectance characteristics of the diffuser panel may change with time on board the spacecraft. Since the diffuser panel is employed in the reference source, any change in the reflectance characteristic distorts the data transmitted back to earth.
A further problem is that, in addition to direct sunlight on the diffuser panel, some light may be scattered from the spacecraft or other instruments on board the spacecraft. The diffuser will then have a higher reflected radiance output which will be interpreted as erroneously low reflectances of earth scenes.
The aforementioned problems are overcome and other advantages are provided by the invention which enables a calibration of the diffuser panel on board the spacecraft as the spacecraft orbits the earth. This is accomplished by comparing intensity of solar radiation (direct and scattered) reflected from the panel with the intensity of the solar radiation (direct only) incident from the sun. The two measurements allow the solar reflectance of the panel to be expressed as a percentage of the incident solar radiation intensity. The ratio, which may be referred to as the calibration ratio, serves as a measure of the panel reflectance. The calibration of the panel is performed when the diffuser panel first enters service as the reference source, and at later times to develop a history of the calibration ratio. The calibration ratio, or data for calculation of the calibration ratio, is transmitted back to earth with the measurement of the earth""s reflected light for correction of the reflectance measurement, thereby to compensate for drift in the diffuser panel characteristics.
The invention is carried out by use of a radiation averaging chamber which may be constructed in spherical shape, and is provided with a diffuse inner reflecting surface to induce multiple reflection of radiation to accomplish an averaging of the radiation. Two ports are provided in a wall of the chamber for entry of radiation, a first of the ports being employed for viewing solar radiation reflected by the diffuser panel, and the second of the ports being employed for sighting solar radiation propagating directly from the sun to the second port. It is advantageous to provide an assembly of baffles encircling the first port to limit the viewing of incoming radiation to only those rays of radiation reflected from the diffuser panel, thereby to exclude any interfering radiation which may be reflected off of other portions of a spacecraft carrying the averaging chamber or from other equipment carried by the spacecraft. Similarly, it is advantageous to encircle the second port with a tubular structure which is pointed towards the sun for receiving only those rays propagating directly from the sun while excluding rays of radiation which may reflect from the spacecraft or equipment carried by the spacecraft from entering the second port.
At least one detector of radiation, such as a photodetector, is located outside the averaging chamber and is optically coupled via a third port to the interior of the chamber for detection of radiation therein. If desired, a lens, such as a fisheye lens may be employed at the third port to facilitate a gathering of radiation to be detected by the detector. Also, if desired, a filter may be located between the lens and the detector to limit radiation incident upon the detector to radiation within a specific portion of the electromagnetic spectrum established by a passband of the filter. Furthermore, if desired, instead of the single photodetector, plural photodetectors each with a different filter may be located at separate ports to detect spectrally different changes in the diffuser reflectance. It is also advantageous to provide shutters at one or both ports for excluding light from the sun during a viewing of radiation from the diffuser panel, or for excluding light from the diffuser panel during a sighting of radiation from the sun.
The foregoing apparatus is employed for calibrating the diffuser panel by sighting the sun via the second port, and detecting the intensity of radiation within the averaging chamber by use of the detector. In the preferred embodiment of the invention, two shutters are employed, namely, a panel shutter positioned in the first port by which the diffuser panel is viewed, and a sun shutter positioned in the second port which serves for sighting the sun. Thereupon, the sun shutter is closed, and the panel shutter is opened to enable the first port to view radiation, which may be light in the visible, ultraviolet and/or infrared portions of the spectrum, reflected from the diffuser panel. The viewing is accomplished by use of the detector(s) each of which detects the intensity of radiation inside the averaging chamber provided by the diffuser panel. Electronic circuitry connected to the detector(s) provides for storing the values of detected radiation. Thereupon, the radiation detected during the viewing of the diffuser panel is divided by the value of radiation detected during the sighting of the sun to obtain a ratio which is useful in calibrating the diffuser panel. An initial value of the calibration ratio is obtained when the diffuser panel is first put into service. Subsequent values of the calibration ratio indicate the presence of a drift in the reflected characteristics of the diffuser panel, or a change in the characteristics of the scattered light. The calibration ratios (or data for calculation of the ratio as will be explained hereinafter) are transmitted along with data of the earth""s reflected light back to a receiving station on the earth. In this way, drift data of the diffuser panel and scattered light changes, in the form of updated values of the calibration ratio can be used to compensate for any drift which may be present in the reflectance characteristics of the diffuser panel, or in the light scattered from the space craft. The compensated values of the diffuser panel reflectances can then be used in correcting the measured values of light reflectances from the earth.
It is advantageous in the use of the averaging chamber to select an aperture size for the second (sun) port which is smaller than the aperture of the first (panel) port, thereby to equalize substantially the amount of radiation power entering the averaging chamber directly from the sun via the second port with the radiant power entering the averaging chamber from the diffuser panel via the first port. This reduces the requisite dynamic range of the detector so as to provide for a more accurate establishment of the calibration ratio.